The correct answer is: A. Blocked entry.
A pure monopolist is the only seller of a good or service in a market. This means that they have no competition and can set their own prices. In the long run, a pure monopolist will be able to make pure profits because they are able to block entry into their market. This means that they are able to prevent other firms from entering the market and competing with them. This allows the monopolist to charge a high price for their goods or services, which results in pure profits.
The other options are not correct because they do not explain why a pure monopolist can make pure profits in the long run. Option B, the high price the monopolist charges, is a result of blocked entry, not a cause of it. Option C, the low LAC costs, is a characteristic of all firms, not just pure monopolists. Option D, advertising, is a way for firms to increase their profits, but it is not a necessary condition for a firm to make pure profits.
In conclusion, the correct answer is A. Blocked entry. This is because blocked entry allows the monopolist to charge a high price for their goods or services, which results in pure profits.